It's frustrating for me that the Nook apps for PC and Android still don't support the Nook "ePIB" format, a variant of ePUB that supports more interactivity such as magnifying and reading aloud passages that are tapped by the reader. When I look at the structure of an ePIB book on my Nook, it seems somewhat like an exploded ePUB file:

I've zipped up the full-page images into a .cbz file so that I can read the book normally with a comic book reader, but it would be nice to be able to get the full interactive experience with a browser plug-in or something.